A main power source (2) is, for example, an ordinary Pb battery and generates a
voltage of 12-13 V. At the time of starting an engine, the main power source (2) supplies power to a
starter (8a). The main power source (2) is given a higher priority than an auxiliary power source (3) to supply power to ordinary loads (8b). The auxiliary power source (3) is a high performance battery (e.g., Li
ion battery), which has superior charge acceptance capability and better state detectability over the main power source (2). Furthermore, the auxiliary power source (3) has an
internal resistance per unit capacity, which is smaller than that of the main power source (2), and generates a
voltage of 9-12 V. A generator (1) is directly connected to the auxiliary power source (3). The auxiliary power source (3) stores regenerative power, which is generated by the generator (1) at the time of deceleration of a vehicle, and is used as a redundant power source for the main power source (2). The main power source (2) and the auxiliary power source (3) are connected to each other through a supply circuit (5), which has a DC /
DC converter (4), and a second supply circuit 7, which has a switch (6).